This invention relates to AC to DC converters and claims the benefit of priority to U.S. Provisional Patent Application No. 60/176,608 titled “IMPROVED AC/DC CONVERTER WITH POWER FACTOR CORRECTION” filed Jan. 18, 2000 and is funded in part by NASA STTR Contract No.: NAS 10-98064.

BACKGROUND AND PRIOR ART

Conventional single-phase rectifier power electronic circuits suffer from high total harmonic distortion (THD) and poor power factor. A number of regulations have been enacted recently to control the harmonic content of line current drawn by the electronic equipment. As a result, researchers have been actively seeking development of power supplies, which can comply with those regulations. In recent years, many circuits and control methods were reported, in which high-frequency switching techniques were used to shape the input current waveform which becomes dominate in power factor correction (PFC). See for example: A. Prasada. P. D. Ziogas and S. Manias. “A Novel Passing Waveshaping Method for Single-Phase Diode Rectifiers.” PESC'89. pp. 99-105; L Barbi and S. A. Oliveira da Silva. “Sinusoidal Line Current Rectification at Unity Power Factor with Boost Quasi-resonant Converters.” In Proceedings of IEEE-APE'90, pp. 553-562; and, P. Kornetzky, H. Wei and L Batarseh. “A Novel One-Stage Power Factor Correction Converter,” IEEE APEC'97 Proc., pp. 251-258.

The implementation of high frequency techniques can be classified into two categories, ie. two-stage scheme and one-stage scheme. In a two-stage scheme, an ac/dc converter with power factor correction is connected to the line followed by a dc/dc converter. These two power stages can be controlled separately, and thus it makes both converters possible to be optimized. The drawbacks of this scheme is lower efficiency due to twice processing of the input power, larger control circuits, higher cost and low reliability.

A one-stage scheme combines the PFC circuit and power conversion circuit in one stage. Due to its simplified power stage and control circuit, this scheme is potentially more efficient. The underline strategy of this scheme is to design the circuit in a certain way that allows its PFC circuit and power conversion circuit to share the same power switch. Several PFC circuits have been reported. See for example: C. Cansem and L Barbi. “A Unity of Power Factor Multiple Isolated Outputs Switching Mode Power Supply Using A Single Switch.” APEC'91. pp. 430-436. These circuits are especially attractive in low cost, low power applications. However, some drawbacks still exist: a) owing to improperly sharing of the power switch, when the converter operates at high frequency, the unavoidable leakage inductance of their power transformers produce high voltage spike at the switching time resulting in decreased efficiency; b) because the power switch performs both PFC and regulation purposes, their regulation capabilities are limited; and, c) at high current and low duty ratio operation, a high voltage presents on the bulk capacitor, resulting in a high rating in design and hence raising the cost. Recently, several single switch converter topologies have been presented to overcome the above drawbacks. See for example: P. Kornetzky. H. Wei and I. Bartarseh. “A Novel One-Stage Power Factor Correction Converter.” IEEE APEC '97 Proc. pp. 251-258; and, Y. S. Lee, IC W. Sui and B. T. Lin, “Single-Stage Isolated Power-Factor-Corrected Power Supplies with Regenerative Clamping.” IEEE APEC '97 Proc. pp. 259-265

U.S. Patents have been recently issued for AC/DC converters with power factor correction but fail to overcome all problems presented above. See for example. U.S. Pat. No. 5,224,025 to Divan et al; U.S. Pat. No. 5,416,387 and U.S. Pat. No. 5,442,539 to Oat et al; U.S. Pat. No. 5,479,331 to Lenni; U.S. Pat. No. 5,510,974 to Gu et al; U.S. Pat. No. 5,515,257 to Ishii: U.S. Pat. No. 5,559,688 to Pringle; U.S. Pat. No. 5,592,128 to Hwang; U.S. Pat. No. 5,594,629 to Steigerwald: U.S. Pat. No. 5,598,326 to Liu et al; U.S. Pat. No. 5,600,546 to Ho et al; and, U.S. Pat. No. 5,619,404 to Zak.

Our recently issued U.S. Pat. No. 5,959,849 dated Sep. 28, 1999 does overcome most of the problems by teaching an AC to DC converter which combines a boost circuit, a Pulse Width Modulation (PWM) switching regulator and a forward circuit power stage in which two storage capacitors are used to relieve the voltage spike produced by the power transformer and to provide energy to the output when the AC line voltage crosses zero. Unfortunately, the rapid evolution of computers and similar digital devices have required power supplies of voltages less than 5 volts and those that function in the hundreds of kilo-herz environments without severe energy losses and resultant failures.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a switching power supply that operates from AC line voltage having a high power factor and output isolation.

The second objective of this invention is to provide a mechanism for soft-switching to allow for a one-stage power factor correction in an AC to DC converter operating in the hundreds of kilo-hertz.

The third objective of this invention is to provide an AC to DC converter having an output transformer that allows the converter to be used for single output and multi-output applications in the low voltage range.

The fourth objective of this invention is to provide an AC to DC converter where the leakage inductance of the forward mode power transformer will not cause an additional voltage stress at the power switch so that a power switch having a lower voltage rating and less power dissipation can be used.

The preferred embodiment of the AC to DC converters with PFC according to this invention is a power supply that provides a DC power to a load from an AC source comprising: a rectifying stage for transferring electrical energy from an AC source into pulsating unipolar voltage pulses at output terminals;

A boost stage having a controllable conducting means and a first unidirectional conduction means for controlling current flow from the output terminals of the rectifying state and blocking the current flow into the opposite direction and a single controllable switching device connected across the output terminals and more than one inductive-capacitive stages preferably two connected between said boost stage and said forward stage providing an inductive energy storing circuit when said switching device is closed and a capacitive charging circuit when said switching device is open, whereby a conversion efficiency of over approximately 75% AC to DC is achieved and with outlet voltage of approximately five volts and lower on alternatively a power on and off switch and an on and off auxiliary switch for controlling both the PFC circuit means and the AC to DC power conversion circuit means, wherein the converter operates at a frequency of greater than about 500 kilo-hertz.

Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated, schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1

is a schematic of the AC to DC converter disclosed and claimed in our U.S. Pat. No. 5,959,849 issued Sep. 28, 1999.

FIG. 2

is a simplified block diagram of the AC/DC low-voltage output converter.

FIG. 3

is the circuit of an AC/DC converter with four-stage switched-capacitor network.

FIG. 4

shows experimental line voltage (upper, 100V/div.) and line current (lower, 1A/div.) waveforms

FIG. 5

shows experimental waveforms under 110VAC input and 8A output (from top, trace

1

: gate signal for the power switch; trace

2

: input inductor current, 2A/div.; trace

3

: drain-source voltage of power switch, 400V/div.; trace

4

: drain current of power switch, 4A/div.)

FIG. 6

shows measured power factor of the AC/DC converter of FIG.

3

.

FIG. 7

shows measured efficiency of the AC/DC converter of FIG.

3

.

FIG. 8

shows measured storage capacitor voltage comparing with calculation at I

0

=12A.

FIG. 9

is a functional schematic of the soft-switching Dual Switch converter unit with PFC and soft switching.

FIG. 10

shows a soft-switching PFC converter with floating auxiliary switch.

FIG. 11

shows waveforms of the soft-switching PFC converter of FIG.

10

.

FIG. 12

shows a

FIG. 10

modified with voltage clamp circuitry.

FIG. 13

a

shows waveforms of the soft-switching PFC converter of

FIG. 12

(recorded under 110VAC input and 110W output): (a) switch waveforms (top: gate signal; bottom: drain-source voltage, 100V/div.)

FIG. 13

b

shows waveforms of the soft-switching PFC converter of

FIG. 12

(recorded under 110VAC input and 110W output): (b) line waveforms (top: line current, 1A/div.; bottom: line voltage, 100V/div.)

FIG. 14

a

shows measurements of the power factor for the converter of FIG.

12

.

FIG. 14

b

shows measurements for the efficiency for the converter of FIG.

12

.

FIG. 15

shows a soft-switching PFC AC/DC converter with grounded auxiliary switches.

FIG. 16

shows steady-state waveforms for the converter of FIG.

15

.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

This invention relates to U.S. Pat. No. 5,959,849, issued: Sep. 28, 1999 to the same inventors and same assignee as the subject invention, which is incorporated by reference and which claims the benefit of priority to U.S. Provisional Patent Application No. 60/050,476 filed Jun. 23, 1997.

In our disclosure of the U.S. Provisional Application Serial No. 60/176,608, several novel converter topologies are set forth which are improvements over the converter topology that has been covered in our previously referenced U.S. Pat. No. 5,959,849 and set forth therein as FIG.

1

. By this reference to U.S. Pat. No. 5,959,849, it is considered that its' text is incorporated fully herein.

FIG. 1

disclosed therein is a single-switch,

106

, converter with Power Factor Correction utilizing a DCM boost circuit as the input stage to provide PFC and a forward circuit as an output staffs to provide electrical isolation. Two storage capacitors,

114

and

116

, have been employed to enhance the PFC capability of the boost circuit and to relieve the voltage spike produced by the power transformer,

118

. The voltages across the storage capacitors,

114

and

116

, are kept at lower levels. The converter has enough line regulation capability to be applied to a universal input.

Referring to

FIG. 1

, converter

1

includes voltage supply VAC.

10

, which can be alternating voltage or current line, that provides sinusoidal voltage with rms value of approximately 120 Volts or another value with a line frequency of approximately 60 Hz. and the like. Output terminals of voltage supply

10

,

10

a

and

10

b

are connected to input terminals

20

a

and

20

b

of any type of an electro magnetic interference (EMI) filter

20

. EMI filter

20

includes storage devices that attenuates high transients of current passing terminals

2

c

and

20

d

on the way to the terminals

10

a

and

10

b

of voltage supply VAC. Output terminals of EMI filter

20

,

20

c

and

20

d

are connected to the input terminals

50

a

and

50

b

of a line rectifier

50

of a conventional type such as KBL06 or any other type consisting of a bridge arrangement of unidirectional conducting devices such as semiconductor diodes in a way that positive voltage of output of line rectifier is delivered to terminal

50

c

connected to ground level

53

of boost/forward (primary) stage

53

. Terminal

50

c

is connected to terminal

102

a

of an inductive device L.

102

, having a value of approximately 420 &mgr;H, and the like.

Terminal

102

b

is connected to terminal

104

a

of a unidirectional conducting device D

1

,

104

such as a fast acting semiconductor diode such as MUR850, and the like. Output terminal of D′,

104

b

is connected positive terminal

106

a

of a controllable switching device

106

, such as a solid state switch such as power MOSFET Type IRF740 or another type of a switching device, and the like. Negative terminal of switching device

106

b

is connected to ground level

53

. Switching device

106

switches on and off with a frequency higher than line frequency and with a ratio of an on and off time that is processed from output voltage at terminal VO,

132

with respect to output ground,

130

is kept constant within small limits. While switching device S,

106

is on. Inductive device L,

102

receives energy from voltage supply VAC,m

10

via EMI filter

20

and line filter

50

. While switching device S

106

is off, energy stored in inductive device L,

102

is mainly transferred to the capacitive devices CS

2

,

116

and CS

1

,

114

via the chain D

1

,

104

, and CS

2

,

116

and D

2

,

112

and CS

1

,

114

and terminal

50

d

and terminal

50

c

to terminal

102

a

of inductive device L,

102

. The capacitances of capacitive devices CS

1

,

114

and CS

2

,

116

have a value of approximately 820 &mgr;F that is great enough to keep the voltages VCS

1

and VCS

2

across capacitive devices CS

1

m

114

and CS

2

,

116

constant within small limits at a value that is always greater than peak value of voltage supply VAC.

10

during normal operation. D

2

is a unidirectional conducting device such as a fast acting semiconductor diode such as V336X and the like. This mode of operation is alike a boost converter and shapes the average current drawn from the power supply VAC.

10

to be of the same shape like the voltage Vline across the power supply VAC.

10

. Referring to

FIG. 1

, forward mode transformer TR

100

consists of two primary windings

110

m

and

118

m

with equal turn numbers and secondary winding

122

. The turn ratio of windings

110

:

118

:

122

is approximately 1:1:0.4. Furthermore leakage inductors L

1

,

108

and L

2

,

120

, have a value of approximately 260 &mgr;H (each) and the like, in serial to primary windings

110

m

and

118

m

respectively.

Referring to FIG.

1

. storage capacitors CS

1

,

114

and CS

2

,

116

are connected to the primary part of forward mode transformer TR,

100

as follows: Terminal

114

a

of capacitive device CS

1

,

114

and terminal

118

c

of leakage inductance L

2

.

118

are connected ground

53

. Terminal

114

b

of capacitive device CS

1

,

114

and terminal

112

b

of unidirectional conducting device are connected together with terminal

110

a

of primary winding

110

m

. Terminal

116

a

of capacitive device CS

1

,

116

and terminal

112

a

of unidirectional conducting device are connected together with terminal

118

a

of primary winding

118

m

. Terminal of

110

c

of leakage inductance L

1

, is connected to terminal

116

b

of capacitive device CS

2

,

116

and to terminal

106

a

of switching device

106

. The terminals

110

b

,

118

a

and

122

a

of forward mode transformer are marked with a dot. They are marked as the beginning of the windings; this means that they always have the same polarity with respect to the terminals

110

a

,

118

b

and

122

b

. While switching device S,

106

, is on energy is transferred from the capacitive devices CS

1

,

114

and CS

2

,

116

to the secondary winding

122

of forward mode transformer TR,

100

. While this happens part of the energy stored in the capacitive devices CS

1

,

114

and CS

2

,

116

is also stored in the leakage inductors L

2

,

110

and L

2

,

418

. These two inductors act as a current limiter for energy transfer. If switching device S,

106

, is off, energy stored in the leakage inductors L

1

,

110

and L

2

,

118

is fed back into the storage devices CS

2

,

116

and CS

1

,

114

as well as into the secondary winding,

122

of forward mode transformer TR,

100

. The secondary winding

122

can be completely isolated from the primary side. RL represents the resistive part of a possible load between the output terminal VO,

132

of converter and output ground

130

. Secondary winding

122

and uniconducting device D

3

,

124

can be an unidirectional conducting device such as a fast acting semiconductor diode such as MUR850 and the like. Components

122

,

124

and capacitive device CL,

126

form a closed loop, where current flow is only allotted towards terminal

124

b

of semi conducting device D

3

. Terminals

122

a

of secondary winding

122

and negative terminal

126

a

of capacitive device CL are connected to output ground level. Capacitive device CL has a value of approximately 900 &mgr;F, that keeps output voltage VO constant within small limits while current ID

3

of uniconducting device D

3

,

124

is not equal to load current IL.

Based on the block diagram of

FIG. 2

four new and improved versions of

FIG. 1

were subsequently developed and disclosed as shown below:

1) Low-Output Voltage AC/DC Converter with PFC Capabilities

This is a single switch AC/DC PFC isolation converter with low-voltage output. The main idea in constructing a low-voltage converter is insertion of such a switched-capacitor step-down network, (

110

/

112

/

114

), in between the input PFC circuit (

102

/

104

′

106

) and the output circuit, (

112

/

24

) shown in FIG.

2

. i.e., more than 1, from 2 to 5 and preferably a total of 4 switched-capacitor step-down networks.

The input circuit,

20

a

,

20

b

, can still remain a boost circuit (

102

/

104

/

106

) operating in DCM to provide near unity input power factor and a dc bus voltage. The switched-capacitor network converts the higher bus voltage to lower level dc voltage when the switch,

106

, is turned on to drive the forward output circuit. Large conversion ratio comes from three approaches: duty ratio control, switched-capacitor step-down network and step-down transformer. It should be noted that the three stages of the converter can share the same power switch.

FIG. 3

shows the proposed converter topology with four-stage step-down switched capacitor network. The forward transformer has four equal-turn primary-windings (N

p1

=N

p2a

+N

p2b

=N

p3a

+N

p3b

=N

p4

=N

p

),

110

, with each of them being connected to one of the capacitors. In order to have equal voltage on the storage capacitors C

1

,

114

, C

2

,

114

s

, C

3

,

114

t

, and C

4

,

114

u

, each primary winding in the inner branches has been divided into two parts. Note that more stages can be used when even low output voltage is desired.

An example 15W-50W 50 kHz converter, was built in the laboratory. The experimental prototype was designed with a 4-stage switched-capacitor network (m=4),

110

,

112

,

114

s

. The transformer,

1

L

2

, turn-ratio n=5 was selected. In the construction of the prototype, the following components were used:

input choke L: 450 &mgr;H. MPP core, identified as

102

Diodes D

1

, D

21

, D

32

and D

43

: MUR840, identified as

112

,

112

s

112

t

,

112

u

, respectively

Diodes

D21

, D

2a

, D

2b

, D

3a

, D

3b

and D

4

: EGP50GL, identified as

112

v

,

112

w

,

112

x

,

112

z

,

112

zz

,

112

zzz

respectively.

Main switch S: IRFP460, identified as

106

.

Output rectifier switches S

01

and S

02

,: F101ON

Forward transformer T: Philips 3C85 ETD-PST39

Storage capacitor C

1

, C

2

, C

3

and C

4

:390 &mgr;F/180V identified as

114

,

114

s

,

114

t

,

114

u

respectively.

Output capacitor C

o

: 470 &mgr;F/10V identified as

126

.

Output inductor L

O

: 100 &mgr;H, MPP core identified as

122

.

PWM IC chip: UC3525A, contains

106

.

The results of the operation with an input voltage of 110 volts @60 Hz were the following output values: V

o

=3.3V±0.5%; Nominal output power: P

o.nom

=30W; Output power range: 10W-50W; and, Switching frequency: f

s

=50 kHz.

Experimental waveforms were recorded by hp54542A oscilloscope.

FIG. 4

shows the filtered input current waveform over line cycle. It can be seen that the input current follows the sinusoidal line voltage very well, implying high power factor at the line side. The detailed input inductor current, drain to source switch voltage and switch current are shown in FIG.

5

. The waveforms were recorded at the peak line voltage. It can be seen that the waveforms agree very well with the theoretical waveforms and the simulated waveforms.

In the experiment, ±20% line voltage variation at 110VAC was considered. Under different load currents, input power factor and efficiency were measured and the measurements were plotted in

FIGS. 6 and 7

, respectively. The measured power factor at 16A load constantly maintains above 98.5%. At light load, the power factor decreased to 97% when the line voltage increased to 135V. The measured overall efficiency of the experimental prototype is around 75% at heavy load. At light load, the efficiency is more sensitive to the line voltage change. The bulk storage capacitor voltage was also monitored in the experiment.

FIG. 8

gives the plot of the capacitor voltage comparing with the theoretical calculation result at

12

A output current. Since the converter belongs to DCM-CCM type, the capacitor voltage increases with the load becoming light.

Novel Soft-Switching Topologies with PFC

The novel soft-switching topologies can be used at much higher switching frequencies than the hard-switching topologies with results of being useful at frequencies in the giga-hertz range with much reduced energy loss.

2) Dual Switch Converter with Soft-switching and Power Factor Corrections

FIG. 9

shows the Dual Switch Converter Unit with Power Factor Correction and Soft Switching converter. The following discusses this operation of the dual switch converter unit with power factor correction (DSCL). The converter consists of a power transformer with four primary windings Lp

1

_A,

3002

, Lp

2

_A,

3004

, Lp

1

_B,

3006

, and Lp

2

_B,

3008

, and

2

: isolated secondary windings Ls

1

,

3010

, and Ls

2

,

2

,

3012

, power switches S_a,

3014

and S_b,

2

,

3016

choke inductors Lchoke_A.

3018

, and Lchoke_B,

3020

, 6 fast rectifiers diodes,

3021

and

3022

, 4 primary storage capacitors Cs

1

_A,

304

, Cs

2

_A,

3026

, Cs

1

_B,

3028

, Cs

2

_B,

3030

, and an output storage capacitor Cout,

3032

. The power switches S_A,

3034

and S_B,

3036

are completed by their parasitic drain source capacitances Cds_A,

3038

, and Cds_B,

3040

. The parasitic leakage inductances of power transformers are represented by L

11

_A,

3042

, L

12

_A,

3044

, L

11

_B,

3046

, and L

12

_B,

3048

.

Assuming the converter operates in steady state mode. All capacitors are charged with their nominal voltages. It can be shown that here are 5 modes of operation described in their order of occurrences:

Mode

1

Lchoke_A,

3018

, and Lchoke_B,

3020

, are discharged and carry no current, S_A,

3034

is closed Current of Lchoke-A,

3018

, increases linearly as wall as of leakage inductances L

11

_A,

30442

and L

12

A,

3044

current of L

11

-A,

3042

, and L

12

-A,

3044

, is transformed to secondary winding Ls

1

,

3010

, and rechargesCout,

3032

, via D

3

,

3022

c

. In that mode energy from power line Vline is stored in Lchoke A,

3018

, and energy from storage capacitors CsI-A,

3024

, and Cs

2

-A,

3026

, is transformed to output loop. Since S-B,

3036

, is open and D

1

-B,

3022

a

, D

2

-B,

3022

b

, and D

4

,

3022

d

are reversed biased, there is no current flow in Lp

1

B,

3006

, md Lp

2

-B,

3008

.

Mode

2

S_A,

3034

, opens and current of Lchoke-A,

3018

, L

11

_A,

3042

, L

12

_B,

3048

form a resonant tank, where Cds_A,

3038

, until Cds_A,

3038

, is charged up to voltage at storage capacitors Cs

2

_A,

3026

, until current of leakage inductances crosses Zero. Because of D

2

-A Voltage across S_A,

3034

, is clamped to VCs

2

-A,

3026

, +VCs I_A,

3024

. During this process voltage across leakage inductances is voltage at storage capacitors+reflected output voltage via Ls

1

,

3010

, /LpI_A,

3002

, and Ls

1

,

3010

, /'Lp

2

_A,

3004

respectively. As soon as the direction of current trough leakage inductances change their sign, converter enters mode

3

Mode

3

L

11

_A,

3042

, LI

2

_A,

3044

, LII_B,

3046

, LI

2

_B,

3048

, Cds_A,

3038

, and Cds_B,

3040

, form a coupled resonant tank, where Cds_A,

3038

and Cds_B,

3040

are discharged simultaneously until Cds_B,

3040

reaches Zero. Because of body diode of Cds_B voltage across Cds-B,

3040

will not increase in negative direction. After Cds_B,

3040

is discharged completely, S_B,

3036

is turned on by the control unit and converter enters Mode

4

.

Mode

4

Mode

4

operates in the same way like Mode I while primary section A and B exchange their operation. After Voltage across Cds_A,

3038

, reaches Zero, converter enters Mode

5

.

Mode

5

operates in the same way like Mode

3

where primary section A and B exchange there operation. If Mode

5

is completed, converter enters Mode

1

again. There are 3 possible methods to control output voltage:

Control Method I.

Output voltage is sensed, compared to a reference voltage and the resulting error controls a PWM controller that operates with a control that is smaller than switching frequency of converter. The PWM-signal switches the control unit on and off.

Control Method

2

Output voltage,

3050

, is sensed and connected to a comparator that switches off as soon as output voltage is larger than a reference voltage and that switches on if sensed voltage is less than reference voltage.

Control Method

3

On time of S_A,

3034

, and S_B,

3036

, is controlled from Zero to maximum value, resulting in variable frequency control of converter.

Since average current of Lchoke_A,

3018

, and Lchoke_B,

3020

, depends next to linearly from line voltage, converter provides power factor correction.

Because voltage across power switches S_A,

3034

and S_B,

3036

are clamped to twice of storage capacitor voltage no additional snubber is required. Since both power switches operate in zero voltage-switching mode, switching losses are reduced even at frequencies above 100 kHz.

Converter operates in complementary mode where energy is transferred during both halfcycles. This reduces the size of power to a minimum.

The topology operates in a forward mode but makes use of the leakage inductance of transformer as an active part. No secondary choke inductor is required compared to classic forward converters. Multiple outputs are possible with this topology.

It should be noted that the two power switches operate at zero-voltage switching with full power transfer. As a result, this converter can operate at 50-100 KHz and possibly higher.

3) Single-Stage Soft Switching ac/dc Converter with Floating Auxiliary Switch

A new soft-switching topology is obtained by adding an auxiliary circuit to the original AC/DC converter as disclosed in FIG.

1

. This original converter circuit cannot achieve high power density due to the fact that the power switch operates under hard-switching with higher than 400V drain-source voltage. In order to create a ZVS condition for the power switch,

1006

, an auxiliary switch S

a

,

1007

, two resonant inductors, L

r1.

1100

, and L

r2

,

1100

a

, a resonant capacitor C

r

,

1114

, and a diode D

r

,

1112

are introduced, shown in

FIG. 5-1

. To relieve the capacitive turn on loss on the auxiliary switch, low output capacitance MOSFET should be considered in selecting S

a

.

Principle of Operation

In each switching cycle, with the auxiliary switch S

a

1007

, turning on a short interval of resonant takes place during which the main switch,

1006

, turns on with ZVS. The cyclic operation of the proposed converter is almost the same as its hard-switching counterpart except that a resonant mode is inserted. The switching waveforms are shown in

FIG. 5-2

and the switching periods are described as follows by assuming that capacitor voltages V

Cs1−

1114

=V

Cs2,

1114

a

=V

Csm.

1114

b

are constant and the line voltage,

1020

, is V

g

in one switching cycle.

Before the auxiliary switch is turned on, the main switch,

1006

, is open, and its output capacitor holds a voltage of 2V

Cs

. The resonant inductors carry zero current. Let's start the cyclic operation when the auxiliary switch is turned on at t

0

.

Switching Period

1

:

At t=t

0

, the auxiliary switch S

a

,

1007

is turned on. A resonance takes place firstly among C

sw

,